1. Field of the Invention
The present invention relates to an efficient compressed domain algorithm to downscale by a factor of k/8 (for any integer k in the range 1≦k≦8) a JPEG digital image or any image/video content that uses the Discrete Cosine Transform (DCT). By chaining multiple such operations, any scale factors of the form k1k2 . . . kl/8l for integers ki's in the range [1,7] can be achieved. The algorithm can be implemented in various apparatuses, methods, and programs of instructions, e.g., software.
2. Description of the Related Art
Traditionally, downscaling a compressed digital image has been done by decompressing the image fully followed by pixel-domain averaging. As such, this approach incurs entropy-decoding, de-zigzagging, de-quantizing, and Inverse DCT (IDCT) costs.
While downscaling in the compressed domain by factors of ½, ¼ and ⅛ has been previously proposed in U.S. Pat. No. 5,708,732, this patent does not teach downscaling by all factors of the form k/8 for 1≦k≦8, including ⅜, ⅝, ¾, ⅞. Moreover, this prior patent does not teach the technique of downscaling by obtaining a k×k block X of pixels from an 8×8 matrix Y of DCT coefficients using the equation: X=(k/8)DkTYkDk, as described in the subject application.